Industrial field of application
The present invention relates to an automatic diaphragm control device of variable photometry type where its objective is for television cameras that are adapted for automatically controlling the objective diaphragm utilizing a video signal supplied from the television camera.
Prior art
As has already been disclosed, for example, in the specifications of U.S. Pat. No. 4,451,851 and U.S. Pat. No. 4,472,743, the conventional television camera utilizing the video signal for automatic diaphragm control is provided with the control circuit having the photometric mode converting function and the sensitivity adjusting function so as to adapt to various shooting environments.
To obtain the optimum picture, in addition to the proper sensitivity adjustment depending on the absolute luminance of an object to be shot, the luminance ratio of bright and dark areas, i.e., the range of contrast is one of the important factors. The range of contrast in the natural world is as high as an order of 10:1 even under clouded sky and often an order of 100:1 under bright sky while the range of contrast which can be represented on the television-reproduced picture is as low as 30 to 40:1. Accordingly, for the picture simultaneously including a bright area and a dark area, it is impossible for a viewer to see both of these areas simultaneously and it must be determined whether the bright area should be selected as the main object to be shot or the dark area should be selected as the main object to be shot.
Theoretically, so-called average photometry mode is suitable for the case in which the dark area is selected as the main object to be shot while so-called peak photometry mode is preferable for the case in which the bright area is selected as the main object to be shot. As the video photometric method of prior art utilizing the video signal coming from the television camera for diaphragm control, any one of the abovementioned types. However, use of such photometric method in a fixed manner has caused various inconveniences. Specifically, for the picture simultaneously including the bright area and the dark area, the average photometry mode has often resulted in the bright area being solidly white or gradientless while the peak photometry mode has often made the dark area, which should be adequately visible in view of the camera sensitivity, almost invisible. With both the peak photometry mode and the average photometry mode, it is required to determine whether the bright area should be selected as the main object to be shot or the dark area should be selected as the main object to be shot, for the scene simultaneously including these bright and dark areas. It is preferred, therefore, that when the object to be shot has no extremely dark area, namely it is in a low contrast condition, the video signal of a same level is obtained even after the photometry mode has been variable adjusted.
Referring to FIG. 12, for example, with a photometric adjuster circuit having its characteristic change from the curve (1) to the curve (2), which provides a steeper gradient of the video signal level, as the photometric mode is variably adjusted from the average photometry mode to the peak photometric mode for a low constrast object, it is assumed that an area ratio of the bright zone to the overall scene is A, a video signal level a covered by the optimum range (shaded zone) has obtained by selecting the characteristic (1). If the area ratio is changed by zooming from A to B, the video signal level would be substantially in excess of said optimum level range and result in an over-luminance. Certainly, the photometry mode may be variably adjusted to the characteristic (2) at this time point to obtain a video signal level b covered also by said optimum range again. However, when the area ratio A is restored by zooming, there occurs an under-luminance a'. To restore the optimum level range a, the photometric mode must be adjusted again to the characteristic (1) or the characteristic (2) must have a sensitivity adjustment to obtain the characteristic (2)'. According to the characteristic curve (2)', an over-luminance b' appears at the area ratio B. Thus, with the photometry adjuster circuit presenting the steeper gradient of the video signal level in the low contrast condition, the selective adjustment between the sensitivity adjustment and the photometric mode adjustment is so difficult that the adjustment may often be disturbed.
To avoid such disturbance, it is desirable that the characteristic video signal level exhibits no substantial variation in the low contrast condition whether the average photometry mode or the peak photometry mode is selected and, therefore, adjustment of the video signal level can be achieved only by the sensitivity adjustment in the low contrast condition while variable adjustment of the photometry mode is effective only in the high contrast condition.